Disruption of ECE-1 and ECE-2 reveals a role for endothelin-converting enzyme-2 in murine cardiac development

Endothelin-converting enzyme-1 and -2 (ECE-1 and -2) are membrane-bound met alloproteases that can cleave biologically the inactive endothelin-1 (ET-1) precursor to form active ET-1 in vitro. We previously reported development al defects in specific subsets of neural crest-derived tissues, including b ranchial arch-derived craniofacial structures, aortic arch arteries, and th e cardiac outflow tract in ECE-1 knockout mice. To examine the role of ECE- 2 in cardiovascular development, we have now generated a null mutation in E CE-2 by homologous recombination. ECE-2 null mice develop normally, are hea lthy into adulthood, are fertile in both sexes, and live a normal life span . However, when they are bred into an ECE-1-null background, defects in car diac outflow structures become more severe than those in ECE-1 single knock out embryos. In addition, ECE-1(-/-); ECE-2(-/-) double null embryos exhibi ted abnormal atrioventricular valve formation, a phenotype never seen in EC E-1 single knockout embryos. In the developing mouse heart, ECE-2 mRNA is e xpressed in the endocardial cushion mesenchyme from embyronic day (E) 12.5, in contrast to the endocardial expression of ECE-1. Levels of mature ET-1 and ET-2 in whole ECE-1(-/-); ECE-2(-/-) embryos at E12.5 do not differ app reciably from those of ECE-1(-/-) embryos. The significant residual ET-1/ET -2 in the ECE-1(-/-); ECE-2(-/-) embryos indicates that proteases distinct from ECE-1 and ECE-2 can carry out ET-1 activation in vivo.